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SAN DIEGO--The open-source Firefox Web browser is critically flawed in the way it handles JavaScript, two hackers said Saturday afternoon.

An attacker could commandeer a computer running the browser simply by crafting a Web page that contains some
malicious JavaScript code, Mischa Spiegelmock and Andrew Wbeelsoi said in a presentation at the
ToorCon hacker conference here. The flaw affects Firefox on Windows, Apple Computer's Mac OS X and Linux, they said.

"Internet Explorer, everybody knows, is not very secure. But Firefox is also fairly insecure," said Spiegelmock, who in everyday life works at blog company SixApart. He detailed the flaw, showing a slide that displayed key parts of the attack code needed
to exploit it.

The flaw is specific to Firefox's implementation of JavaScript, a 10-year-old scripting language widely used on the Web. In particular, various programming tricks can cause a stack overflow error, Spiegelmock said. The implementation is a "complete mess,"
he said. "It is impossible to patch."

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The hackers claim they know of about 30 unpatched Firefox flaws. They don't plan to disclose them, instead holding on to the bugs.